Configuration Conformational property

Discussion on any kind of polymers is logically preceded by at least a quick survey of properties of parent monomers. This is particularly relevant in the case of polysaccharides because monosaccharides are rather special monomers, with peculiar configurational-conformational properties and reactivity. Monosaccharides are multifunctional compounds, usually with three or four OH groups of approximately equal chemical reactivity reducing sugars have, of course, additional reaction capabilities. 

For the configuration-conformational properties of simple carbohydrates, as well as their reactivities, comprehensive treatises should be consulted (Pigman and Horton, 1972). It has to be mentioned on closing, that recent achievements in macromolecular science have established the ways to the synthesis of linear homopolysaccharides and copolysaccharides (normally by cationic polymerization) of different suitably protected anhydrosugars (Schuerch, 1981). These syntheses open the route to the availability for our studies of a variety of tailor made carbohydrate polymers. A few synthetic polysaccharides have in fact already been used to investigate immunological and allergic reactions in experimental animals and the interaction of plant lectins with carbohydrates (Schuerch, 1981). 

C NMR spectroscopy of 1,7-dioxa- (314), l-oxa-7-thia- (315), and 1,7-dithiaspiro[5.5]undecane (316) and many of their derivatives, with special emphasis on stereoelectronic and steric effects governing the configurational and conformational properties of these compounds (484-486). This work showed that chemical-shift parameters developed for six-membered carbocyclic compounds in chair conformations (149) could be applied successfully only to sulfur and not to oxygen heterocycles (486). 

The reaction of iV-acyloxy-iV-alkoxyamides and aromatic amines, the thermal decomposition of iV,iV -diacyl-iV,iV -dialkoxyhydrazines as well as the base-induced decomposition of azodicarboxyiates are examples of the HERON reaction, one of the newest-named reactions in the chemical literature , but one that seems to be common for these as well as a range of other bisheteroatom-substituted amides . No doubt new examples will be discovered and it is entirely predicated upon the configurational and conformational properties of anomeric amides, attributes arising directly as a consequence of the dual electron demands of the heteroatoms at nitrogen. 

Li, X.-C. et al.. Absolute configuration, conformation, and chiral properties of fiavanone-(3 8")-flavone biflavonoids from Rheedia acuminata. Tetrahedron, 58, 8709, 2002. 

Finally, it could be asked why it is so important to properly identify stereogenic centers and prefer them to helical units The main practical reason is that most molecules, when realistically described, e.g., by a crystal structure, contain numerous helical units and it is exceedingly difficult, particularly with a computer program, to sort out those that are invariant to physical conditions (see Section 1.1.1.). In general, it is advisable to base specification of stereogenic units on constitutional or configurational distinctions of ligands, as these are normally less affected by external conditions than conformational properties. 

This section discusses some simple chemical reactions which convert a chiral nonracemic compound containing no electron absorption band in an easily accessible spectral range into a derivative with absorption in either the visible or quartz ultraviolet region. This is a useful operation if a reliable correlation exists between absolute configuration (conformation) and chiroptical properties. A collection of useful chromophoric systems is found in reference 167. 

Thus, a brief survey of the current understanding of the molecular and super-molecular structures of common thermoplastics is presented first. This review starts with a brief description of the current state-of-the-art knowledge of the constitution, configuration, conformation and supermolecular structure of common glassy and semicrystalline thermoplastics. Later in this chapter, specific features of the structure-property relationships are discussed in greater detail for the most frequently filled thermoplastics. Effects of fillers on the structural variables in polypropylene, considered the most commercially important matrix, are especially emphasized. 

The first family is represented by peptides based on a, 3-dehydro a-amino adds in which a C=C bond confers rigidity and electronic Y-conjugation on the system (Section 11.1). Peptides rich in these amino acids are widespread in nature, particularly as antibiotics, and possess interesting conformational properties. 1-11 In addition to the E and Z configurational isomers, occurring when two different atoms (substituents) on CP are present, fully-extended, characteristically flat conformations or p-tums and 310/a-helices may be induced by u,p-dehydro a-amino acids. 

In a typical analysis of a polymer chain, the experimental values of configuration-dependent properties and their temperature coefficients are compared with the results of rotational isomeric state calculations. These comparisons yield values of the energies for the various rotational states about the backbone bonds, and these conformational preferences can then be used to predict other configuration-dependent properties of the chains. It is also possible to obtain such conformational information from potential energy calculations, using the methods of molecular mechanics.39,46 52... 

The large number of theoretically acceptable duplexes described above suggests that DNA may adopt a variety of closely related helical structures in solution. A conformational blend of such helices within a single complex offers a more realistic interpretation of configuration-dependent properties of DNA in solution than the various rigid molecular models currently in use (27, 28). Chemical exchange studies (29, 30, 31, 32, 33) indicate, however, that the DNA duplexes do not remain perfectly... 

In this chapter, the configurational, conformational and dynamic properties of mono- and oligosaccharides will be discussed and, in general, reference is made to reviews that cover these aspects. These... 

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